The present invention relates to an electron energy loss spectrum measuring apparatus, a transmission electron microscope or a scanning transmission electron microscope, and an electron energy loss spectrum measuring method.
As semiconductor devices and magnetic head elements become small and microscopic, the elements has a structure where thin films of several nm (nanometer) are laminated in an area of about submicron. Since the structure, the element distribution, the crystal structure, and the chemical bonding state of the micro area largely affect characteristics of the semiconductor elements and the magnetic head elements, it is important to analyze the micro area.
Methods for observing the micro area include a scanning electron microscope (SEM), a transmission electron microscope (TEM), and a scanning transmission electron microscope (STEM). The TEM and the STEM have a spatial resolution at nanometer level. The TEM is an apparatus which irradiates an electron beam almost parallel to a specimen, and magnifies the transmitted electron beam with a lens or the like. On the other hand, the STEM converges an electron beam into a micro area, measures the transmitted electron beam while scanning the electron beam in two dimensions on a specimen, and obtains a 2 D image.
An energy loss specific to an element (electron structure) is generated by an interaction with the elements constituting a specimen when an electron beam transmits through the specimen in the TEM and the STEM. Electron energy loss spectroscopy (EELS) uses an electron spectrometer to apply an energy analysis to electrons which have transmitted through the specimen, and is an analyzing method which can analyze elements in the specimen. A difference in chemical bonding state of identical elements especially reflects the electron structure of the element, and is appears as an energy shift at a level of a few eV. As a conventional analyzing apparatus, the TEM or the STEM combined with an electron energy loss spectrometer (EELS) of parallel detection type is widely used.
An electron beam transmits through a specimen, passes through an objective lens, a projection lens, and an incident aperture, and enters into the EELS in the STEM. The EELS has such a structure that a magnetic sector in a fan shape serves as an electron spectrometer, a quadrupole electromagnetic lens and a hexapole electromagnetic lens are provided front and behind of it, and a parallel type electron beam detector is provided most downstream. The quadrupole electromagnetic lens is used for adjusting a focus of an electron energy loss spectrum, and magnifying the electron energy loss spectrum. The hexapole electromagnetic lens is used to reduce an aberration of the electron energy loss spectrum projected on the electron beam detector. The electron energy loss spectrum magnified by the quadrupole electromagnetic lens is projected on the electron beam detector, and the electron energy loss spectrum spanning a wide range is measured.
The electron beam detector comprises a scintillator receiving an electron beam and emitting fluorescence, and an element comprising multiple pixels for receiving the fluorescence. Alternately, it is a detector comprising multiple pixels for receiving an electron beam. The electron energy loss spectrum is measured based on the incident fluorescence or electron beam intensity.
Prior art relating to the structure of the EELS includes U.S. Pat. No. 4,743,756, Japanese application patent laid-open publication No. Hei 07-21966, Japanese application patent laid-open publication No. Hei 07-21967, and Japanese application patent laid-open publication No. Hei 07-29544.
A user has to repeat an operation comprising (1) specifying a location to be measured, (2) specifying an element or an energy range to be measured, and (3) using an EELS to measure an electron energy loss spectrum at points to be measured for a conventional analyzing apparatus constituted by combining an STEM with an EELS. Alternately, a location to be measured is specified before hand, and the location is stored in a controller for controlling electron beam scan, and conducts operations (2) and (3) described above. There used to be a problem that a pixel position changes when a trajectory of the electron beam changes because of an effect of an external electromagnetic field, thereby degrading energy precision and accuracy of the electron energy loss spectrum.
The degradation of the energy precision and accuracy of the electron energy loss spectrum causes the following problems when the electron energy loss spectrum at a specific location or multiple locations is measured on a specimen as described above.
(A) FIG. 3(b) shows an example where the spectrum precision degrades when an electron energy loss spectrum is measured at a specific position on a specimen. For example, when an electron energy loss spectrum caused by an inner shell electron excitation of oxygen is measured, the measurement takes one second, and a pixel position for coming into the electron beam detector deviates from a reference pixel position (dotted line) because of an effect from an external electromagnetic field generated in this period in the figure. A spectrum indicated in (i) of FIG. 3(b) is detected for 0.5 second from the beginning of the measuring. The energy shifts because of the effect of the external magnetic field as indicated in (ii) of FIG. 3(b) at 0.5 second. In this case, a spectrum after measuring for one second becomes an electron energy loss spectrum shown in (iii) of FIG. 3(b). As the result, a peak shape of oxygen is wide, and a peak position shifts.
(B) FIG. 4(c) shows an example where the spectrum accuracy degrades when the electron energy loss spectrum is measured at multiple locations on a specimen. For example, a specimen has a structure of laminating a material A (constituting element: A), a material with an unknown constituting element (constituting element is assumed to be B), and material and material C (constituting element C), and the individual materials are measured in a sequence of the material A, the material B, and the material C as shown in FIG. 4(a). The materials A, B, and C respectively present peaks at energy positions (a), (b), and (c) indicated in dotted line on the electron energy loss spectrum in FIGS. 4(b) and (c) if there is no effect from an external electromagnetic field or the like. Even if a component element is an unknown material B, it is determined that the material is composed with the element B according to the energy peak position. When a measurement is conducted sequentially from the material A, if the energy shifted because of an external electromagnetic field while an electron beam is maintained on the material B during measuring, the spectrum of the material B shifts as shown in FIG. 4(c). When the element from the material B is identified based on spectra at individual measured points, there is a problem that a false result is obtained.
Prior art for solving this problem includes U.S. Pat. No. 5,798,524 and Japanese application patent laid-open publication No. 2000-113854. However, they cannot solve the problem described in (B).
Thus, it is essential to measure an electron energy loss spectrum after energy correction for solving the problem describe above.
The purpose of the present invention is to provide an apparatus and a method for measuring electron energy loss spectrum at high precision and high accuracy with an apparatus combining a TEM or an STEM with an EELS.
The present invention provides an EELS apparatus provided with an electron beam detector which comprises multiple pixels, and measures a spectrum of an electron beam which has transmitted through a specimen, and a controller which controls the position of the electron beam incident to the electron beam detector such that the electron beam detector measures a spectrum with a know dispersion, a position deviation pixel number between a pixel position for a peak appeared on the spectrum measured by the electron beam detector, and a reference pixel position designated as a reference position on the electron beam detector is measured, the position deviation pixel number is converted into a control factor for controlling the position of the electron beam such as a voltage value or a current value based on the spectrum dispersion, and the position deviation is corrected based on the control factor.
The electron energy loss spectrum measuring method of the present invention features that after an operation for correcting the peak position deviation of the spectrum is conducted with the EELS apparatus provided with the peak position control apparatus for correcting a peak position in a spectrum, the electron energy loss spectrum is measured at high precision.
In addition, the electron beam detector detects zero-loss peak where the spectrum intensity is at the maximum, and the controller corrects such that the peak position of the zero-loss peak matches a reference peak position of the electron beam detector, thereby correcting spectrum energy at high precision in a short period, resulting in measuring an electron energy loss spectrum at high precision and high accuracy. Even if the zero-loss peak does not exist on the electron beam detector, the controller can control such that the zero-loss peak appears on the electron beam detector, and then the controller can again control such that the zero-loss peak matches the reference pixel position of the electron beam detector. Thus, the energy correction takes a short period, and the measuring is conducted at high precision and at high accuracy when a wider range of electron energy loss spectrum is measured.
An STEM or a TEM provided with the EELS according to the present invention has a controller comprising a memory for storing a result of spectrum measuring by the electron beam detector, a data base for storing a database for core loss energy or plasmon loss energy for elements to be analyzed, measuring conditions and the like, and a central controller for controlling the spectrum measuring and peak position control operation. In addition, the STEM provided with the EELS has a controller comprises a memory for storing a spectrum measured by the electron beam detector, an energy filter controller for controlling the peak position on the electron beam detector, and an STEM controller for controlling the electron beam position on a specimen.
An operation is repeated such that a spectrum is measured after the energy correction, then measuring is switched to another location, and again, a spectrum is measured after the energy correction in the electron energy loss spectrum measuring method of the present invention where locations for measuring are changed sequentially on a specimen. With this method, the electron energy loss spectrum is measured at high precision and at high accuracy when multiple locations are measured on a specimen.
The EELS apparatus of the present invention includes at least two electron beam detectors comprising an electron beam detector for detecting a zero-loss to correct energy, and an electron beam detector for measuring an electron energy loss spectrum.
The present invention allows automatically adjusting the focus of a spectrum.